(dailyRx News) According to the National Institute of Mental Health, depression is the leading cause of disability in the U.S. for people ages 15 to 44. In any given year, this disorder affects some 14.8 million people age 18 and older, or about 6.7 percent of the U.S. population.
Fortunately, depression is highly treatable and people suffering from it can get better. The most common treatments for depression are one or more antidepressant drugs, counseling or a combination of the two. Frequently prescribed antidepressants include citalopram (Celexa), fluoxetine (Prozac), paroxetine (Paxil) and sertraline (Zoloft). According to research firm IMS Health, sales of selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs), the two leading types of antidepressants, reached $9.6 billion in the U.S. in 2008.

Because a number of antidepressants are available in the U.S., there's a fair amount of confusion about the individual drugs and about the antidepressant medication category itself. To help resolve the confusion and give both health care providers and patients a better understanding of medication options, the Agency for Healthcare Research and Quality (AHRQ), part of the U.S. Department of Health and Human Services, has reviewed dozens of clinical trials, research reports and other data and put forth some information that consumers ought to know about medications for treating depression.

Fast Facts About Antidepressants
According to the AHRQ, six out of 10 people feel better with the first antidepressant their health care provider prescribes. The remaining four must try other antidepressants until they find the one that's right for them. This processing can take some time because it typically takes at least six weeks for a person taking an antidepressant to begin to experience the medication's full effect.

Another potential problem with finding the "right" antidepressant is a medication's side effects. Most people who take antidepressants, the AHRQ found, experience at least one side effect from their depression medication. Predicting which antidepressant will cause which side effect for any patient is difficult, and obviously a debilitating side effect overrides the intended effect of the antidepressant, which is to help a person feel and function better.

The most common side effects of antidepressants are constipation, daytime sleepiness, diarrhea, dizziness, dry mouth, headache, nausea, sexual difficulties, shakiness, trouble sleeping and weight gain. Often, these side effects go away after a few weeks of taking the medication, but some side effects only go away if the patient stops taking that particular medication. However, stopping a medication cold turkey has serious consequences and should never be done. Instead, a patient who is experiencing side effects from an antidepressant (or any other medication) should taper off the drug with the guidance of the health care provider who prescribed the medication.

Most people prescribed an antidepressant take it for six to nine months. Some people must take them longer.

Antidepressants and Their Common Side Effects
As noted before, most people taking an antidepressant medication experience at least one side effect. Some medications on the market are more likely to cause certain side effects than others. From its review of existing data, the AHRQ has compiled a review of top-prescribed antidepressants and their more probable common side effects.

Weight gain. People who take mirtazapine (Remeron) are more likely to gain weight than people taking: who take one of these medications:

• Citalopram (Celexa)
• Fluoxetine (Prozac)
• Paroxetine (Paxil)
• Sertraline (Zoloft)

The weight gained while taking mirtazapine is around two to seven pounds, although some people can gain much more.

Depression patients prescribed paroxetine (Paxil) are more likely to gain weight than patients who take fluoxetine (Prozac) or sertraline (Zoloft). The AHRQ found that for someone who weighs 150 pounds and takes paroxetine, the average weight gain is about five pounds. That said, some people can gain much more if taking this antidepressant.

Conversely, people who take bupropion (Wellbutrin) tend to lose a little weight--about two to three pounds.

Nausea. vomiting and diarrhea. Venlafaxine (Effexor) appears to make a depression patient slightly more likely to experience nausea and vomiting than these antidepressants:

• Citalopram (Celexa)
• Esciltalopram (Lexapro)
• Fluoxetine (Prozac)
• Fluvoxamine
• Paroxetine (Paxil)
• Sertraline (Zoloft)

In fact, the AHRQ found that venlafaxine as well as paroxetine are more likely than the other antidepressants to cause nausea, vomiting and other flu-like symptoms if a patient suddenly stops taking the drug. However, fluoxetine is less likely than other antidepressants to cause these symptoms when stopped suddenly.

More people (8 percent more) experience diarrhea when taking sertraline (Zoloft) than with the medications:

• Bupropion (Wellbutrin)
• Citalopram (Celexa)
• Fluoxetine (Prozac)
• Fluvoxamine
• Paroxetine (Paxil)
• Mirtazapine (Remeron)
• Nefazodone
• Venlafaxine (Effexor)

Sexual difficulties. Some antidepressants can cause sexual difficulties, such as loss of sexual desire or loss of the ability to reach orgasm. The AHRQ found that taking paroxetine (Paxil) put a patient at a higher risk of experiencing sexual difficulties than these medications:

• Fluoxetine (Prozac)
• Fluvoxamine
• Nefazodone
• Sertraline (Zoloft)

On the other hand, bupropion (Wellbutrin) appeared to present less of a risk for sexual difficulties than these antidepressants:

• Fluoxetine (Prozac)
• Paroxetine (Paxil)
• Sertraline (Zoloft)

While side effects can diminish over time, they can persist, rendering a medication an unviable option for the patient. Any depression patient should let his or her health care provider know about any side effects and the duration of those side effects he or she experiences when taking an antidepressant. With this knowledge, the prescriber can offer the patient other options.

Rare but Serious Side Effects of Antidepressants
The two leading classes of antidepressants, selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs), increase the amount of the neurotransmitter serotonin in the brain. Raising serotonin levels is beneficial--up to a point.

Too much serotonin can cause serotonin syndrome. This rare, but serious drug reaction occurs most often when a person takes simultaneously two drugs that raise serotonin levels in the brain. In addition to SSRIs and SNRIs, other drugs that raise serotonin include the following:

• Migraine medications called triptans (Amerge, Imitrex)
• Dietary supplement L-tryptophan
• Herbal products such as St. John's wort
• Dextromethorphan, an ingredient in some over-the-counter cough medications
• Prescription pain relievers such as meperidine (Demerol)

Common symptoms of serotonin syndrome include confusion, hallucinations, loss of coordination, fever, rapid heart rate and vomiting.

One simple step can help people taking antidepressants reduce their risk of serotonin syndrome: talking about their medications with their health care provider and pharmacist. Both the prescribing health care provider and the dispensing pharmacist can provide guidance for avoiding serotonin syndrome so along as she or he knows what other prescription or over-the-counter medications, dietary supplements and herbal products the patient is also taking.

In addition to serotonin syndrome, people who take antidepressants can be at increased risk for suicide. This is especially true if the patient is younger than 25. Research is ongoing to try to determine which of these medications might put a person at higher risk for suicide and suicidal thoughts. But no matter what their age, depression patients should immediately alert their health care provider if their depression gets worse or they begin having suicidal thoughts.

The Unknowns of Antidepressants
While antidepressants have been used for a number of years, there is still a lot of information health care providers don't know about these medications. For example, not much is known about the long-term effects and consequences of using these drugs for a long time. That's because research studies investigating them typically last only three months to one year.

Another unknown is how the more frequently prescribed antidepressants compare for treating symptoms that often accompany depression, such as insomnia or chronic pain. Additionally, it's not known if any particular antidepressant works better for women than men.

While many questions remain about how to effectively prescribe and make use of antidepressants for the variety of people who experience depression, those unknowns should not stop a person who is suffering from the disorder from seeking medical help.

More information about depression and its treatment options from the National Institutes of Health and the U.S. National Library of Medicine is available at www.nlm.nih.gov/medlineplus/depression.html. Information about mental health services and resources by state are available online at mentalhealth.samhsa.gov/databases/ from the Center for Mental Health Services, part of the U.S. Substance Abuse and Mental Health Services Administration.